When linking a library libA with a library libB using libtool, if libB.la
exists, libtool will add all libraries libB.la refers to (dependency_libs
field) to the linker command line and store them in the dependency_libs
field of libA.la. So everything that subsequently links with libA will also
link to these extra libraries. This causes too much overlinking.
This commit modifies Mk/Uses/libtool.mk so it empties the dependency_libs
field in .la libraries during staging. However, because .la libraries have
very limited use when dependency_libs is empty it makes sense to completely
remove them during staging.
So with this commit USES=libtool is modified to remove .la libraries and a
new form (USES=libtool:keepla) is introduced in case they need to be kept
(dependency_libs is still emptied).